FIG. 1 illustrates one general solar cell of mass production model using a mono- or polycrystalline silicon substrate. The solar cell includes a silicon substrate 101 in which an impurity is diffused at a high concentration to form a diffusion layer 102 and a p-n junction, and electrodes on its light-receiving surface. The electrodes include a multiplicity of electrodes (referred to as extraction electrodes) 104 of several hundreds to several tens of microns (μm) wide, and several collector electrodes 105 as the electrode for integrating the extraction electrodes together and interconnecting solar cells. While these electrodes may be formed by various methods, one method commonly employed from the standpoint of cost is by printing a metal paste comprising fine particles of metal such as Ag and an organic binder through a screen or the like, and heat treating at a temperature of several hundred degree centigrade for bonding to the substrate. On the surface of the substrate opposite to the light-receiving surface, a back electrode 106 of opposite polarity to the light-receiving side electrode is formed by using a metal paste comprising fine particles of metal such as Al and an organic binder, screen printing, and firing at a temperature of about 700 to 850° C. In the region where light is incident on the solar cell, an antireflective film 103 is formed for efficiently taking in light. A silicon nitride film which is formed by chemical vapor deposition (CVD) or the like is commonly used as the antireflective film.
The antireflective film also has a further important function of passivating the silicon surface. In the interior of crystals, silicon atoms are in a stable state due to the covalent bond between adjacent atoms. However, at the surface corresponding to the terminus of atom arrangement, where no adjacent atom to be bonded is available, an unstable energy level known as “dangling bond” appears. Since the dangling bond is electrically active, it captures and extinguishes charge photogenerated within silicon, detracting from the operation of a solar cell. To suppress the loss, the solar cells have been subjected to surface passivating treatment or otherwise treated to reduce dangling bonds.
On the other hand, it is known that dangling bonds are not passivated at the interface where metal and silicon contact, and a recombination rate of carriers is very high thereat. That is, an electrode must be contacted with the silicon surface for extracting photogenerated carriers whereas the silicon/electrode interface becomes a significant loss component to solar cell characteristics. Therefore, for high-efficiency solar cells, some approaches are taken to minimize the contact area between silicon and electrode. Such approaches include narrow contact and point contact structures. These structures are formed by partially removing the passivation film by photolithography (see, for example, J. Knobloch, A. Noel, E. Schaffer, U. Schubert, F. J. Kamerewerd, S. Klussmann, W. Wettling, Proc. the 23rd IEEE Photovoltaic Specialists Conference, p. 271, 1993) or etching paste printing, to expose the underlying silicon, and evaporating or printing a metal thereon. An alternative method is by forming a metal film on the passivation film, irradiating laser light as spots thereto to heat the metal, and letting the metal penetrate through the passivation film, for thereby forming silicon/electrode contacts (see, for example, S. W. Glunz, R. Preu, S. Schaefer, E. Schneiderlochner, W. Pfleging, R. Ludemann, G. Willeke, Proc. the 28th IEEE Photovoltaic Specialists Conference, p. 168, 2000).